This Laboratory has historically been involved in the study of tissue lesions mediated by the formation of chemically reactive metabolites of drug and other foreign compounds. A method has been developed to estimate in vivo the intrinsic clearances of enzymes that catalyze the formation of short-lived reactive metabolite. The basis for this method is that at low doses of the parent compound, the reactive metabolite preferentially reacts with endogenous substances such as glutathione (GSH). The depletion of GSH can be served as an index for the formation of the reactive metabolite. The assumptions made in this method are 1) the concentration of hepatic reactive metabolite reaches a steady state almost instantaneously, 2) the rate of the formation of reactive metabolite follows first order kinetics, and 3) the rate of the formation of the GSH conjugate follows second order kinetics. The animal model for this study was male hamsters. A marginally toxic dose of acetaminophen was used. The intrinsic clearance for the formation on chemically reactive metabolite was estimated from the rate of synthesis of hepatic GSH (calculated), the fraction of the dose of acetaminophen converted to GSH conjugate and reported values of hepatic blood flow. This intrinsic clearance for reactive metabolite formation was errified by measuring the rates of formation of the GSH conjugate in the 9000 x g supernatant fraction of hamster liver.